The behavioral and neuroscientific literature on children and adolescents with autism spectrum disorder (ASD) has grown dramatically over the past decades. In contrast, almost nothing is known about ASD later in life. This is problematic in view of preliminary findings suggesting that some older people with ASD may experience accelerated cognitive and neurological decline, combined with extensive evidence of age-related complications and decline in other neurodevelopmental disorders. The present study will be the first systematic investigation of brain anatomy and connectivity in mid to late adulthood in ASD. We will recruit 100 adults with ASD, ages 45-65 years, and 80 matched neurotypical control (NC) participants, and will use anatomical, functional connectivity, and diffusion MRI as well as a battery of sensorimotor and cognitive tests in a longitudinal study, with participants returning after 2.5 years for a second time point. Specific aims will be to examine (1) brain anatomy (cortical thickness and volumes of gray matter structures, white matter, and CSF) and (2) anatomical and functional brain connectivity in adults with ASD compared to NC participants, (3) to characterize cognitive and behavioral abilities, and correlate these skills with brain structure and function, and (4) to create an extensive inventory of variables that may predict long- term neurocognitive success or decline in ASD. Analyses will be performed for a broad range of domains and networks, with some focus on executive, motor, and memory systems. Whereas aims 1-3 will provide a currently unavailable systematic body of evidence on changes in brain and behavior during mid to late adulthood in ASD, aim 4 will have strong prospective translational relevance because identification of brain sites and behavioral measures most likely to predict age-related decline vs. successful aging in ASD will be crucial for the development of preventative interventions.